1. Field of the Invention
The present invention relates to an efficient method of coding a video image capable of decreasing the quantity of information per pixel of digital video signals.
2. Description of the Prior Art
Various methods of coding video signals of this sort have been known hitherto, including, for example, the high efficiency technique for coding a digital video signal disclosed in U.S. Pat. No. 4,703,352, and the block separated component progressive coding reported by Alain Leger et al. in "Still Picture Compression Algorithms Evaluated for International Standardization," CH2535-3/88/0000-1028 1988 IEEE. The former method is explained below.
In the first place, a block is formed by grouping together a specific quantity of pixel data of input digital video signals (for example, one block consists of 16 pieces of pixel data within a rectangular region obtained by dividing a screen in every 4 pixels in the horizontal direction, and every 4 pixels in the vertical direction). Detecting the maximum value MAX and the minimum value MIN in the pixel data (the data expressing the gray scale) within a block, the dynamic range DR (the difference between the maximum value and the minimum value) of every block is determined, and the pixel data within a block is respectively quantized and coded by using the quantizing level provided within the dynamic range (a range from the minimum value to the maximum value).
The pixel data within a block strongly correlate with each other, and the mutual difference is often a small value. Therefore, the dynamic range in the block is smaller than the dynamic range determined by the word length of the video data (the video signal is generally quantized in 8 bits, and its dynamic range is the difference between the maximum quantizing level of 255 and the minimum quantizing level of 0, i.e. 255), and by setting the quantizing level only in the dynamic range, the number of bits to be quantized in the pixel data can be reduced. Furthermore, in a block having a wide dynamic range such as an edge portion of a picture, deterioration is not obvious even if quantized coarsely, and hence the number of bits to be quantized may be further decreased. On the basis of this principle, an efficient coding is realized in this method.
The quantizing level when quantizing within the dynamic range in n bits is the median value of a range of, for example, dividing the dynamic range equally into 2.sup.n pieces. Since the quantizing level is created from the maximum value and the minimum value, transmitting of any two of the maximum value, minimum value, and dynamic range as side information in every block allows the same quantizing level setting to be reproduced at the decoding side, so that decoding is possible. When the number of bits n is constant, the pixel data is quantized in the quantizing step size proportional to the dynamic range.
In transmitting two pieces of side information per block and quantized data of n bits per pixel, supposing the number of bits of input video data to be 8, the number of quantized bits n to be 3, and the number of pixels in a block to be 16, since the maximum value, minimum value and dynamic range can output in each block is EQU 8+8+3.times.16=64 bits.
By contrast, the total number of bits of the input in every block is EQU 8.times.16=128 bits.
Therefore the video data quantity is compressed to 1/2.
The problem is, however, that further enhancement of coding efficiency is being demanded.